Method of manufacturing a luminescent material

ABSTRACT

The invention relates to a method of manufacturing europium-doped (Ca1-xSrx)S (0£×£1) luminescent material with a short decay time and a high thermal extinction temperature, wherein the europium-doped strontium sulfide is subjected to at least a first caldnation step at high temperatures in the presence of at least one iodine compound. The invention further relates to the luminescent material as such and to its use for light-emitting components such as light-emitting diodes (LEDs) and laser diodes coated with luminescent materials.

The invention relates to a method of manufacturing a europium-doped(Ca_(1-x)Sr_(x))S (0<x<1) luminescent material with a short decay timeand a high thermal extinction temperature, to the luminescent materialitself, and to its use in light-emitting components such aslight-emitting diodes (LEDs) and laser diodes coated with luminescentmaterials.

Sulfates, carbonates, oxalates, or oxides are generally used as basicmaterials for manufacturing alkaline earth sulfide fluorescent powdersin the prior art. High temperatures of more than 900° C. are necessaryfor the manufacture of such powders so as to reduce oxygen-containingbonds to the corresponding sulfide compounds and to achieve as completeas possible a distribution of activators and co-activators in the hostlattice.

Three different methods of manufacturing alkaline earth sulfidefluorescent powders are known in the prior art; for a general summarysee: Ghosh and Ray, Prog. Crystal Growth and Chart. 25 (1992) 1):

-   -   1. reduction of alkaline earth sulfate with hydrogen,    -   2. sulfurizing of alkaline earth carbonate or oxide with H₂S or        CS₂,    -   3. sulfurizing and melting method, this is a modified version of        the industrial process for manufacturing rare earth metal oxide        sulfide phosphors.

The method mentioned third is based on the alkali-polysulfide meltingmethod by means of which very well crystallized phosphor particles areobtained, as is described by Okamoto et al. in U.S. Pat. No. 4,348,299.This method, however, has several disadvantages for the manufacture ofSrS:Eu luminescent materials. Thus a molten mass is usually obtainedafter calcination, which is to be washed with an aqueous solution so asto dissolve the recrystallized alkali polysulfide melt. The methodmentioned can be very well used in the case of a calcium sulfidephosphor, because this material is stable in aqueous surroundings. Thisis not true, however, for materials comprising strontium sulfide,because these are not stable in aqueous surroundings, so that the methodis unsuitable for this.

A further disadvantage is that an excess of alkali atoms is present inthe host lattice, so that these alkali acceptors are to be compensatedfor equalizing the charge. This is achieved, for example, by oxidationof Eu(II) to Eu(III), which is accompanied by a strong reduction in thedesired Eu(II) emission, as represented below:Na₂S+2Sr_(Sr)+2Eu_(Sr)→2Na_(Sr)′+2Eu_(Sr)+2SrS   (1)

The crystallinity of alkaline sulfide fluorescent powder manufactured byone of the methods mentioned sub 1) or 2) above may be improved by anadditional calcination step and by the use of a flow promoting agent,for example ammonium chloride or ammonium bromide, as described by Yocomand Zaremba in U.S. Pat. No. 4,839,092 for NH₄X (X═Cl, Br).Ammoniumchloride and bromide readily react with sulfide compounds, afterthermal dissociation during calcination, whereby the correspondinghalogen compounds are formed, while a reducing atmosphere is created bythe evolving NH₃, as shown below:2NH₄X+SrS→2NH₃+H₂S+SrX₂   (2)

The strontium halide SrX₂ has a much lower melting point than strontiumsulfide, so that a liquid phase is formed during the heating step,surrounding the SrS particle. A dissolution and recrystallization of thestrontium sulfide at the solid-liquid boundary surface leads to a graingrowth of the particles and to an improved particle morphology. Inaddition, well-crystallized particles and a good particle morphology areimportant factors which are decisive for the efficiency of theluminescent properties of the material, especially if the excitationwave line lies in the visible spectral range.

The incorporation of halogen atoms into the strontium sulfide hostlattice during the calcination step leads to the creation of positivecharge defects in the anion sub-lattice, which is compensated by cationvoids:SrX₂+2S_(S)+Sr_(Sr)→2X_(S)·+V_(Sr)″+2SrS   (3)

These charge lattice defects act as electrons and holes, so that astrong afterglow of the above luminescent material is obtained afterexcitation. This effect may be utilized for the manufacture of strontiumsulfide phosphor with a long afterglow, as described in U.S. Pat. No.4,839,092. A disadvantage of fluorescent materials with such a longafterglow and with such a high density of defects is that they have astrong thermal extinction of the luminescence, i.e. a strong decrease inthe luminescent power at increased temperatures. Such materials areaccordingly not suitable for most lighting applications.

Koichi and Akira, Japan Pat. No. 60,101,172 describe a method ofimproving the afterglow properties and the brightness of europium-dopedstrontium sulfide by means of a thermal treatment of the luminescentmaterial with an alkaline earth metal vapor under a given vaporpressure. A major disadvantage of this method is that alkaline earthmetal vapors are toxic and exhibit a very high reactivity with mostmaterials in the reaction chamber. This method is accordingly notsuitable for industrial mass manufacture of luminescent materials.

It is an object of the present invention to provide a method ofmanufacturing highly effective, europium-doped (Ca_(1-x)Sr_(x))S (0≦x≦1)with short luminescence decay times and a high thermal extinctiontemperature, while the above disadvantages of the prior art are avoided.

According to the invention, a europium-doped (Ca_(1-x)Sr_(x))S (0≦x≦1)luminescent material with a short decay time and a high thermalextinction temperature can be manufactured in that europium-doped(Ca_(1-x)Sr_(x))S (0≦x≦1) is exposed to at least a first calcinationstep at high temperatures in the presence of at least one iodinecompound.

In the method according to the invention, the (Ca_(1-x)Sr_(x)S:Eu,I)(0≦x≦1) luminescent material should be calcinated at least once in areducing atmosphere.

Suitable reducing atmospheres are formed by an inert atmosphere, such asargon or nitrogen, which comprises sulfur, preferably sulfur inelementary form.

It was found to be advantageous to add small quantities of hydrogen tothe inert atmosphere so as to prevent an oxidation of the luminescentmaterial, in particular during calcination.

The europium dopant is present as a cation and the iodine as an anion inthe lattice of the (SrS:Eu,I) luminescent material.

It is advantageous when the europium-doped (Ca_(1-x)Sr_(x)S:Eu,I)(0≦x<1) luminescent material comprising iodine, i.e. in the form ofiodine ions I⁻, is subjected at least to a second calcination step athigh temperatures, preferably in the presence of a reducing atmosphere.

The afterglow period can be shortened and the brightness can beincreased in that the luminescent material is crushed, for example in aball mill, and is subsequently subjected to a calcination step.

The temperatures of the calcination step or steps may be ≧900° C. in themethods used according to the invention. The temperatures preferably liein a range from 950° C. to 1500° C., preferably 1050° C. to 1200° C.

In a preferred embodiment of the method according to the invention, theluminescent material is fired in an inert atmosphere containing sulfur,preferably 2 to 4% of sulfur by weight, possibly in the presence ofsmall quantities of hydrogen.

Preferably, the quantity of added europium lies between 0.001 and 0.5atom %, preferably between 0.005 and 0.2 atom %, with respect to theCa_(1-x)Sr_(x)S (0≦x<1).

To promote the crystal growth of the europium-doped Ca_(1-x)Sr_(x)Sparticles (0≦x≦1), at least one iodine compound, preferably chosen fromthe group comprising I₂ vapor, ammonium iodide (NH₄I), strontium iodide(SrI₂), calcium iodide (CaI₂), magnesium iodide (MgI₂), zinc iodide(ZnI₂), and/or barium iodide (BaI2), is added.

The proportion of added iodine compounds should lie in a range ofbetween 0.1 and 5 atom %, preferably in a range of between 0.5 and 4atom %, and preferably in a range of between 1 and 3 atom %, withrespect to the Ca_(1-x)Sr_(x)S (0≦x≦1).

After calcination of the luminescent material, the iodine anion contentof the luminescent material according to the invention should be ≦5000ppm, preferably ≦1000 ppm, more preferably ≦500 ppm, even morepreferably ≦300 ppm, highly preferably ≦200 ppm, and most preferably≦100 ppm. The lower the proportional quantity of iodine anions in theluminescent material according to the invention, the better luminescentproperties are observed for the luminescent material according to theinvention. After calcination of the luminescent material according tothe invention with iodine anions, the iodine anion content of theluminescent material according to the invention should ideally be asclose to zero as possible.

It is preferred according to the invention that 2 atom % of ammoniumiodide is calcinated together with the Ca_(1-x)Sr_(x)S:Eu (0≦x≦1) andwith 2 to 4% by weight of sulfur in a loosely closed, argon-filledcorundum tube at temperatures of between 1050° C. and 1150° C. for 1 to2 hours in a nitrogen flow. The use of a corundum tube is advantageousfor keeping hydrogen iodide, which is formed in the thermal dissociationof ammonium iodide, in the reaction zone so that the hydrogen iodidethus formed reacts with the strontium sulfide, forming a temporaryliquid phase at the particle surfaces.

After this heating step, Ca_(1-x)Sr_(x)S:Eu,I (0≦x≦1) luminescentmaterial exhibits a strong afterglow. The afterglow can be shortened andthe brightness can be increased in that the luminescent material iscrushed, for example by means of a ball mill, followed by a final firingor calcinating step in a reducing nitrogen atmosphere, preferably alsocontaining sulfur, for 1 to 2 hours at temperatures of 950° C. to 1050°C.

This subsequent second calcination step renders it possible to removemost lattice defects of the luminescent material, i.e. iodine anionatoms in sulfur atom locations and strontium cation atom defects orCa_(1-x)Sr_(x) cation atom defects, while in addition surface defects ofthe particles are restored again.

SrS:Eu,I luminescent material emitting in the visible wavelength rangeof 610-620 nm, i.e. in the orange color wavelength range, andCa_(1-x)Sr_(x)S:Eu,I (0≦x≦1) luminescent material emitting in the610-655 nm wavelength range can be obtained by the method according tothe invention as described above. The higher the Ca content of theCa_(1-x)Sr_(x)S:Eu,I (0≦x≦1) luminescent material, the more thewavelength range is shifted to greater wavelengths.

The absorption of the Ca_(1-x)Sr_(x)S:Eu,I (0≦x≦1) luminescent materiallies in a range from 350 nm to 500 nm, depending on the Ca content.

The method according to the invention renders it possible tomanufacture, for example, SrS:Eu,I luminescent material which has theproperties listed in Table I below. TABLE I Quantum efficiency (T = 20°C., λ_(exc) = 460 nm) >90% Absorption at λ = 440-470 nm >75% Luminousefficacy 260 Im/W Color point x = 0.626, y = 0.370 1/10 Afterglow decaytime (λ_(exc) = 460 nm) <0.7 ms Thermal decay (T = 20-200° C.) <7%Average particle size <15 μm

The strongly luminescing, europium-doped Ca_(1-x)Sr_(x)S:Eu,I (0≦x≦1)materials comprising iodine anions, as manufactured by the methodaccording to the invention, have the following advantages overeuropium-doped Ca_(1-x)Sr_(x)S (0≦x≦1) luminescent materialsmanufactured in accordance with the prior art:

-   1. the use of an iodine-sintered flowing agent for manufacturing    luminescent europium-doped Ca_(1-x)Sr_(x)S material comprising    iodine ions yields optimized particles with a high degree of    absorption in the blue spectral range and a high conversion    efficiency. The material manufactured in accordance with the    invention is accordingly particularly suitable for color conversions    in blue LEDs.-   2. Compared with prior-art europium-doped strontium sulfide    materials calcinated with bromine or chlorine compounds, leading to    luminescent materials with long decay periods, the material    according to the invention can be subsequently processed in a    reducing atmosphere, preferably in a nitrogen atmosphere containing    sulfur, without further measures, whereby a material of high    efficiency, a short decay time, and a high thermal extinction    temperature can be obtained. The latter is a result of the short    decay time of the luminescence, which is an important characteristic    for a suitable color converter for a lighting means, such as LEDs or    laser LEDs coated with the luminescent material according to the    invention, because the operating temperatures of an LED chip will    exceed 200° C. in the near future.-   3. The decay time of the materials according to the invention is    even shorter than the time reported for SrS:Eu materials known from    the prior art, which are calcinated in the presence of a strontium    metal vapor.

It should be noted, furthermore, that the heating ofCa_(1-x)Sr_(x)S:Eu,I (0≦x≦1) according to the invention in a reducingatmosphere, in particular a nitrogen atmosphere containing sulfur, is amethod that can be readily implemented on a large scale, whereas this isnot possible for a method in which the luminescent material is exposedto a strontium metal vapor, because this method requires speciallydeveloped, expensive reaction chambers made from non-reactive materials.

The luminescent material according to the invention has a high thermalextinction temperature. In particular, at T=20° C. to 200° C., said highthermal extinction temperature amounts to ≦20%, preferably ≦15%, morepreferably ≦10%, highly preferably ≦7%, and most preferably ≦5%.

The luminescent material according to the invention may thus beadvantageously used as a luminescent means, preferably as a coating ofluminescent material on lighting means.

Lighting means in the sense of the present invention comprise inparticular also light-emitting components, liquid crystal picturescreens, electroluminescent picture screens, fluorescent lamps,light-emitting diodes, and laser diodes coated with the luminescentmaterial according to the invention.

The subject of the present invention will be explained in more detail bymeans of the manufacturing examples 1 and 2 given below, without beinglimited thereto.

General notes on the experimental arrangement for the manufacture ofSrS:Eu,I according to the invention:

To manufacture SrS:Eu, a tubular firing chamber comprising a corundumtube was used, through which nitrogen with 1% of hydrogen by volumeadded thereto was made to flow. The europium-doped strontium sulfidemixed with ammonium iodide and sulfur was introduced into two aluminumoxide boats. Each boat was placed in an argon-filled corundum tube andmoved to the hottest spot during calcination.

EXAMPLE 1

Manufacture of SrS:Eu,I

Solution A

230.84 g Sr(NO₃)₂ (99.99% purity) was added to a mixture of 750 ml twicedistilled H₂O and 1 ml of a concentrated aqueous solution of (NH₄)₂S.The solution was filtered through a 0.45 μm filter after 24 hours(solution A).

Solution B

157.89 g (NH₄)₂SO₄ (99,99% purity) was added to a mixture of 750 mltwice distilled H₂O and 1 ml of a concentrated aqueous solution of NH₃.The solution was filtered through a 0.45 μm filter after 24 hours(solution B).

Solution A+Solution B

The two solutions A and B were slowly joined together under stirring in0.5 1 water-free alcohol. The SrSO₄ precipitate formed thereby waswashed with twice distilled H₂O and then dried. Subsequently, 0.486 gEu(NO₃)₃.6H₂O was dissolved in little water and stirred together withSrSO₄ into a paste. After drying, the europium-coated SrSO₄ was crushedinto a powder and heated in air for one hour at 500° C. Then the sulfatewas converted into sulfide by heating in a reducing gas atmosphere of 5%H₂ by volume and 95% N₂ by volume during 12 hours at 1000° C. and asubsequent heating during 4 hours in the reducing gas atmosphere underaddition of dry H₂S. The SrS:Eu thus formed was milled into a powder ina ball mill after the addition of cyclohexane, and subsequently the drypowder was mixed with 3.0 g NH₄I (99.99% purity) and 10 g sulfur (99.99%purity). The mixture was put in an aluminum oxide boat and thenintroduced into a loosely closable, argon-filled corundum tube andheated for one hour at 1100° C. in a flow of nitrogen. Any inert gas maybe used instead of argon. The luminescent material SrS:Eu,I was thenwashed with water-free methanol, dried, and milled for 30 minutes in aball mill in cyclohexane. The resulting SrS:Eu,I powder was once morecalcinated in a nitrogen flow containing sulfur for 1.5 hours in aloosely covered aluminum oxide boat in a corundum tube at 1000° C. Theresulting SrS:Eu,I luminescent material was subjected to an ultrasonictreatment in water-free ethanol for 15 minutes, dried, and sieved (meshsize 45 μm).

EXAMPLE 2

Manufacture of Ca_(1-x)Sr_(x)S:Eu,I (0≦x≦1)

Various Ca_(1-x)Sr_(x)S:Eu,I luminescent materials (0≦x≦1) were preparedby the method described in example 1, with the proviso thatCa_(0.25)Sr_(0.75)S, Ca_(0.5)Sr_(0.5)S, and Ca_(0.75)Sr_(0.25)S wereused instead of SrS.

1. A method of manufacturing europium-doped (Ca_(1-x)Sr_(x))S (0≦x≦1)luminescent material with a short decay time and a high thermalextinction temperature, characterized in that europium-doped(Ca_(1-x)Sr_(x))S (0≦x≦1) is exposed to at least a first calcinationstep at high temperatures in the presence of at least one iodinecompound.
 2. A method of manufacturing a luminescent material as claimedin claim 1, characterized in that the europium-doped (Ca_(1-x)Sr_(x))S(0≦x≦1) luminescent material comprising iodine ions is subjected atleast to a second calcination step at high temperatures.
 3. A method ofmanufacturing a luminescent material as claimed in claim 1,characterized in that the temperatures of the calcination step are ≧900°C., preferably in a range from 950° C. to 1500° C., more preferably1050° C. to 1200° C.
 4. A method of manufacturing a luminescent materialas claimed in claim 1, characterized in that the luminescent material issubjected to at least one calcination step in a reducing atmosphere,preferably an inert atmosphere containing sulfur, particularlypreferably an inert atmosphere containing 2 to 4% by weight of sulfur.5. A method of manufacturing a luminescent material as claimed in claim1, characterized in that the iodine anion content of the luminescentmaterial is between ≦0 and ≦5000 ppm, preferably ≦1000 ppm, morepreferably ≦500 ppm, even more preferably ≦300 ppm, highly preferably≦200 ppm, and most preferably ≦100 ppm.
 6. A luminescent material havingthe composition (Ca_(1-x)Sr_(x))S:Eu,I (0≦x≦1).
 7. A luminescentmaterial as claimed in any claim 1, characterized in that theluminescent material has a short decay time, preferably with a 1/10afterglow decay time for λ_(exc)=460 nm being <0.7 ms.
 8. A luminescentmaterial as claimed in claim 1, characterized in that the luminescentmaterial has a high thermal extinction temperature, in particular saidhigh thermal extinction temperature at T=20° C. to 200° C. amounting to≦20%, preferably ≦15%, more preferably ≦10%, highly preferably ≦7%, andmost preferably ≦5%.
 9. A lighting means, characterized in that saidlighting means comprises a luminescent material as claimed in claim 1,preferably a coating of luminescent material.
 10. A lighting means asclaimed in any one of claim 1, characterized in that the lighting meansis a light-emitting component, a liquid crystal picture screen, anelectroluminescent picture screen, a fluorescent lamp, and/or alight-emitting diode.